Nuclear imaging and therapy are vital to several areas of modern medicine, including oncology, cardiology, hematology, and studies of the biodistribution of drugs. These non-invasive techniques rely on the introduction of radioactive agents (radiopharmaceuticals) into the body to detect disease via Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT), or to treat disease with ionizing radiation. To minimize the systemic exposure of the body to radiation, and to enhance the specificity and sensitivity of the PET and SPECT imaging techniques, metallic radionuclides (radiometals) are bound to biomolecules (BMs) with bi-functional chelators (BFCs). The BM (e.g., a peptide or an antibody) is selected to have a high affinity for the tissue of interest, to deliver and retain radiation only where it is needed. The BFC is selected to have a high affinity for the radiometal, form a complex with the radiometal that is highly stable in vivo, and possess a functional group that can form a bond with the biomolecule.
Two radionuclides commonly used in nuclear imaging are the positron emitter 18F (used in PET), and the gamma ray emitter 99mTc (used in SPECT). These radionuclides have relatively short half-lives (109 minutes and 6 hours, respectively) that make them favorable for minimizing exposure of the body to radiation, and have decay characteristics that make them optimal for their respective imaging modalities. However, focusing on PET imaging, the relatively short half-life of 18F and its typical labeling conditions (use of organic solvents) lowers its suitability for use with biomolecules such as antibodies. An alternative radionuclide that has received increasing attention is the positron emitter 64Cu2+. The decay properties of this radiometal allow it to be used both as a PET imaging agent and as a nuclear therapy agent. In addition, its half-life of 12.7 hours and the capability of cyclotron-based production of large quantities with high specific activity (>10,000 mCi/mol) from enriched 64Ni3 facilitate the distribution of 64Cu2+ from a central production facility. Further benefits associated with 64Cu2+ include: (1) its well-documented coordination chemistry, redox chemistry, and biochemistry and metabolism in humans, (2) the availability of a variety of azamacrocyclic BFCs that can chelate copper in the 2+ oxidation state with high specificity and stability, and (3) the ability to perform radiolabeling reactions in protein-friendly, aqueous media (as opposed to the organic solvents required for radiolabeling with 18F), at pH ˜7, and at near-physiological temperatures.9 
Conventional radiolabeling methods for 64Cu2+, and other radiometals, typically require the dilution of small quantities (1-2 mCi≈4 picomoles of 64Cu2+ in ˜10 μL is diluted to ˜500 μL) for convenient handling and proper mixing, resulting in nanomolar concentrations of the radiometal. This dilution requires a large excess (˜100-fold) of the potentially expensive and difficult-to-obtain BFC-BM conjugate to ensure the desired high percentage of bound radionuclide (>90%) within a reasonable time (<1 hour). In turn, the use of large excesses of BFC-BM conjugate necessitates extensive chromatographic purification to remove unlabeled BFC-BMs and to obtain the high specific activities that are desirable for application of the radiopharmaceutical, for example in PET imaging. Chromatographic purification is also potentially required to remove BFC-BM impurities that may bind more strongly or more quickly to the radiometal than the desired BFC-BM conjugate. For instance, if the 100-fold excess of BFC-BM contains 1% impurity, then the molar ratio of impurity to radiometal would be 1:1, potentially leading to the synthesis of unwanted radiometal-ligand complex.